Making small lenses

Arthur Wilkinson

Bob Neville’s detailed description of the work involved in making a mirror not only reminded me of the many similar jobs I have undertaken, but also revived a memory of what is a puzzle to me: why do many mirror-makers shy away from making small lenses? I would therefore like to describe what is involved in a relatively easy job. As I am now 80 years of age, my story will necessarily be historical but, like home mirror-making by hand, the technique has not changed that much since I did it – unless one is lucky enough to have a diamond-impregnated turning tool.

During the Second World War, F.J. Hargreaves discovered that a then obsolete photographic lens known as a ‘telenegative’ – particularly one made in the early years of the century by the German firm of Goerz – could make an excellent Barlow. Having bought all these lenses that he could find just after the war ended, Jim (as he was known to his friends) talked me – an innocent youngster but with a few mirrors to my credit – into making some. Horace Dall had dismantled one of the German lenses and provided us with a specification: that is, radii and focal length (63 mm) on the assumption that the glasses were common types.

Before describing the work, I should say that the German design, being intended for a very different use, was not only much larger than necessary but was not the easiest to make, while it proved better to use it the other way round as a Barlow. The surfaces comprised a flat front and, being a doublet, the cemented surfaces were almost hemispherical, while the last surface had a knife-edge rim that had to be flush with the edge of the other component. After making about a dozen I had to give up for domestic reasons, and I then redesigned the lens on similar lines but with just one seventh of the amount of glass.

Having no other use for my design, I gave it to Ron Irving, who had it made up by Emerson Optical and, having found it worked well, later told me that he had sold hundreds! My reward from Hargreaves – who tested and sold the lenses that I made – was the verdict that they were better than the original; but being hand-made, perhaps they should have been! A footnote to this is that, many years later, when my son showed me a Japanese teleconverter lens that he had bought for his camera, it appeared (as far as I could tell) to be an exact copy of my modified Barlow, arranged in reverse form to what it should have been for that purpose. Despite having had no workshop training, but possessing all three volumes of Amateur Telescope Making, I plunged into the job by going to a shop in Euston Road that catered for model-makers – where I bought two lengths of ½-inch silver-steel rod, an assortment of plummer blocks and some pulleys and belts for comparatively little outlay – and then to Gamages, where I purchased a ¼-h.p. electric motor. I had a basement room to work in, and it did not take long to set up vertical and horizontal spindles, while a friend with a lathe threaded the ends of the shafts and made curved ‘tools’ for me. As brass rod of the right size was then difficult to acquire, the tools were made of mild steel and worked very well. (I was amused some twenty years later when on a conducted tour of the West German Zeiss works at Oberkochen, to hear the guide proudly proclaim that they had just made a great discovery: that mild steel tools work better than any others.)

After that it was relatively easy, although I had bought the optical glass from Chance’s in 5-inch square blocks for cheapness and had to cut out the 1¼-inch discs with a brass tube. I cut through three blocks at a time – which was quite rapid with 80 carborundum. I held the lenses in my fingers while surfacing them and, with suitable speeds on the spindles, progress was equally rapid. I had made a crude but effective spherometer with a brass disc, four 2BA nuts and bolts and an old penny drilled and soldered to the central bolt, making me guilty of the crime of defacing the coinage. After the usual sequence of grinding and polishing it remained only to edge the components, which I did by cementing the lenses to a chuck on the horizontal spindle with pitch and holding a brass strip against the edges. As the chuck was not of high quality there was some initial chattering, but it worked quite well. I spoiled only one component by chipping the knife-edge, but then made a smaller-diameter Barlow from it.

For anyone with a reasonable workshop it should be much easier to make small lenses on a lathe. but I would not think that it is to be recommended. With the steep curves that I was using I could work only one component at a time, but with flatter curves several pieces could be cemented on a larger tool to speed up the job considerably. Of course, the principal argument against making one’s own small lenses is simply that it is much quicker, and little if no dearer, to buy them ready made. Unless large quantities are made there is little profit in it; but I found it an interesting, if at times frustrating, experience.